Methods of this type for detecting the start of a dive by using various physical principles are known from the prior art.
EP Patent No. 0 689 109 granted in the name of CITIZEN WATCH CO. LTD. On 16 Dec. 1998, discloses a method of this type, and a portable electronic device for implementing the same. In particular, this device is provided with specific means arranged for detecting when the device enters into contact with water, and a pressure sensor, arranged for measuring the value of the ambient pressure. According to this Patent, the pressure sensor is powered, in a first operating mode, so as to carry out an atmospheric pressure measurement once an hour in order to store the value thereby obtained as the reference pressure value. Moreover, the specific water detection means, which can for example take the form of ohmic contacts arranged on the case of the device, are permanently or periodically powered.
The ohmic contacts thus fulfil a main switch function for the circuits dedicated to the operating mode relating to diving in particular for the pressure sensor. Indeed, when the presence of water is detected at the ohmic contacts, the supply frequency of the pressure sensor is altered such that the ambient pressure measurements are carried out with a period of the order of a second, within the scope of an operating mode called the preparation mode. These measurements allow the value of the pressure variation between the last measured value and the last reference pressure value stored to be calculated, the value of the variation then being compared to a predefined value forming a dive mode trigger threshold. When the pressure variation breaches the trigger threshold, the dive mode is activated. In the opposite case, the sensor is still powered for several minutes to monitor the development of the ambient pressure. Once this time period has elapsed, the preparation mode is deactivated and the pressure sensor is again powered with a period of the order of an hour.
Such a detection method enables a distinction to be made, for example, between a situation in which the person wearing the device wets it by washing his hands and a situation actually corresponding to the start of a dive. In the latter case, the pressure measurements carried out by the pressure sensor enable the start of a dive to be validated after activation of the preparation mode, insofar as the device undergoes contact with water first of all before seeing the ambient pressure increase.
However, this type of device has a significant drawback from the point of view of its construction, arising from the need to provide specific means for detecting the presence of water on the case. In the aforecited case using ohmic contacts, it is in fact imperative that specific means are provided to guarantee that the case of the device is water tight in the area of such contacts, which can have significant consequences as regards the manufacturing cost of the device. Consequently, the method described hereinbefore has a similar drawback because it is based on the implementation of specific means for detecting the presence of water.
Other methods and devices are known from the prior art which do not implement such specific means for detecting the presence of water and which use ambient pressure measurements to detect the start of a dive.
In particular, devices of this type are known wherein a pressure sensor is periodically powered to measure the value of the ambient pressure, the results of such measurements being stored. These devices are arranged such that at each new ambient pressure measurement, the value of the variation between this last measurement and the previous one is calculated and compared to a value defining a trigger threshold. Once the trigger threshold has been breached, the dive mode is activated, the second to last pressure value measured being typically stored as a reference pressure, i.e. it is assumed to correspond to the surface pressure of the body of water in which the dive takes places.
This type of device has, however, a drawback because the accuracy of detection of the start of a dive is entirely based upon the value retained for the trigger threshold.
Thus, if the value retained for this threshold is too low, the device is exposed to a risk of the dive mode being inadvertently triggered. By way of example, if the person wearing such a device comes down a mountain road at a sustained rhythm, the device is capable of considering the corresponding increase in pressure similar to entry into water. On the other hand, if the value retained for the threshold is too high, the trigger precision is liable to be poor if the user stays in the water in proximity to the surface for some time before the start of a dive. In such a case, it is also possible for the stored reference pressure value to be incorrect since it was measured in the water, at a lower depth than that of the trigger threshold. Such an error can, depending on its amplitude, have dangerous consequences for the health of the person wearing the device, particularly from the point of view of data relating to a decompression stop onto which the error would be carried.